3 Week Warning for Deadly Heat Waves

Heat waves kill more Americans every year than hurricanes, tornadoes, floods, earthquakes and lightning combined, according to the Centers for Disease Control. A recently identified atmospheric pattern may give people a two- to three-week warning that a deadly heat wave threatens to engulf the United States.

This map of air flow a few miles above ground level in the Northern Hemisphere shows the type of wavenumber-5 pattern associated with US drought. This pattern includes alternating troughs (blue contours) and ridges (red contours), with an “H” symbol (for high pressure) shown at the center of each of the five ridges. High pressure tends to cause sinking air and suppress precipitation, which can allow a heat wave to develop and intensify over land areas. (Haiyan Teng, National Center for Atmospheric Research)

National Center for Atmospheric Research scientists observed a particular sequence of five alternating low and high-pressure systems in the atmosphere far above the North Atlantic. When that pattern moved over the continental United States, it increased the chances of a heat wave.

The five high-pressure systems ringed the mid-latitudes of the north like pearls on a necklace moving slowly westward. When that pattern emerged, the United States faced double to quadruple the chance of a serious heat wave after approximately 20 days. The pattern blocked movement in the atmosphere and allowed hot, dry conditions to take hold. For example, the pattern preceded the heat waves and droughts of 1952-54 and 1988.

To discern the pattern, the atmospheric scientists used weather records and atmospheric measurements since 1948. A computer simulation of 12,000 years of U.S. weather supplemented the historical observations.